A: Echolocation can be broadly described as “seeing
with sound”. As early as 1793, Italian researcher Lazzaro Spallanzai
demonstrated that, while blinded bats could find their way around their
enclosure, deafened bats lost their sense of direction. The term
“echolocation” was first coined by the late Harvard zoologist Donald R.
Griffin who, back in 1938, used a microphone sensitive to ultrasound to
listen to bats. Echolocation is effectively the ability to localize
(find) objects based on how they reflect sound. In the case of our
visual system, we rely on light reflected back from objects around us in
order to see – bats rely on sound reflected back from objects around
them in order to “see in the dark”. Bats emit a variety of chirps and
squeaks during flight and listen for the echoes. Sound striking close
objects will be reflected back sooner and be louder than sound striking
a more distant obstacle. Similarly, by listening for changes in the
phase of the echo, bats can determine the type of surface from which the
sound was bounced back – a hard, continuous object (such as a wall) will
produce a sharper echo than softer objects (such as foliage).

The figure (left) shows the basic principle of echolocation – a sound is
produced, bounces back from the first object (the moth in this example)
and then, a fraction of a second later, bounces back from a second,
third, fourth etc. object (e.g. trees, walls, hedges, etc. in the
vicinity). If the bat knows how fast this ‘block’ of sound is
travelling, it can calculate -- based on the time separating the two
returning echoes -- the distance between the two objects. Moreover, the
bat can vary the harmonics, rate, length, intensity and components of
the call to gain an extraordinary amount of information about its
surroundings.

The frequency -- measured in kilohertz (kHz), or thousand’s of cycles
per second -- of bat calls varies with species and, it is generally
considered that, high-frequency sounds give the bat lots of detail but
over a short distance, while low-frequency sounds give less detail but
over a longer range. Although the rare Short-eared Trident bat (Cloeotis
percivali) of South Africa can call at frequencies as high as 212 kHz --
bearing in mind that humans can only hear sounds as high as about 20 kHz
-- frequencies of between 20 and 60 kHz are more common. Frequencies
lower than about 20 kHz have a wavelength larger than most insects (so
the sound wave moves around the insect, rather than striking it and
bouncing back) while frequencies above 60 kHz attenuate (weaken) rapidly
in air, which lessens their range.

Bat calls can generally be classed into two groups: narrowband and
broadband. Narrowband calls (sometimes referred to as Constant
Frequency, or CF, calls) are those of almost constant frequency, while
broadband calls (sometimes referred to as Frequency Modulation, or FM,
calls) sweep a large range of frequencies in a very short time (i.e.
from 100 kHz down to 20 kHz in a couple of seconds). Broadband calls
are used to scan the landscape, while narrowband calls are used to
identify and provide information on potential prey items. The search
calls tend to be intense (with some 10 to 15 calls -- or pulses -- every
second in some species), getting faster and faster (up to 200 calls per
second) to home in once an insect has been detected.

In his New Encyclopedia of Mammals, David Macdonald points out that
bats tend to have a “Duty Cycle” when echolocating, which represents the
proportion of time actually spent generating the sound. In other words,
bats only spend a certain period of their time (e.g. about 20%) echolocating, because they can’t listen for returning echoes whilst
shouting new pulses. This is not true of all bats; Horseshoe bats (Rhinolophidae)
can apparently shout and listen at the same time, allowing them to spend
as much as half their time echolocating. This is achieved through
something known as “Doppler Shift Compensation” or DSC. The example I
was always given at school to explain the Doppler Shift was the ‘train
approaching a station’ illustration – we hear a high pitched sound as a
fast moving train approaches the station platform (because the sound
waves are being compressed by the approaching train) and a lower pitched
sound as the train races past and away from the station (because the
sound waves are being stretched). Thus, the Doppler Shift may be
described as the change in pitch (i.e. frequency) of a sound produced by
a moving object. How the bats use this DSC is actually rather
complicated and involves varying their call frequency according to their
flight speed. Sufficed to say that Horseshoe bats send (as ultrasound)
and receive (as echoes) sounds on different bandwidths, enabling them to
separate their outgoing, echolocative, calls from the returning echoes.

The bat larynx (voice box) is large and reinforced with bone,
allowing a high tension on the vocal chords to be maintained (permitting
the production of high-frequency vibrations). According to A.A. Wardhaugh’s
1995 book, Bats of the British Isles, sounds are produced in
concentrated beams that are directed through a gap in the upper incisors
of most species. Horseshoe bats are slightly different – rhinolophids
have a flap of skin adorning their nose, which is used to concentrate
the stream of sound pulses (in a similar way to a megaphone). This flap
of skin seems to afford the Horseshoe bats extra sensitivity, allowing
them to detect insects at distances of some 10m (30 ft), while the vesper
(or "evening") bats can only detect insects at distances of about 1m
(almost 3.5 ft). Once the call has been emitted, the ear muscles relax
and await the returning echo. Many microbats have a ‘spike’ of cartilage
sticking up from the base of their ear, which scientists believe help
give the bat better sound detection in a given plane. The echolocation
of bats is impressively accurate. An intriguing paper, by Sabine Schmidt
at the University of Munich in Germany and two colleagues in October
2000, found that Gleaning bats (Megaderma lyra) were able to find silent
and motionless prey on the ground and use their broadband echolocation
calls to reject dummy food items whilst hovering over them.

It seems that the ability to echolocate is largely a characteristic
of the microbats; megabats (Fruit bats, or Flying foxes), with few
exceptions, don’t echolocate because they have sufficiently good vision
to find fruit by sight (scent is probably also involved). One exception
to this is the Egyptian Fruit bat (Rousettus aegyptiacus), which
apparently uses echolocation to find its way about in caves. Indeed, a
study by Dean Waters at the University of Leeds and Claudia Vollrath at
the University of Freiburg in Germany, found that R. aegyptiacus used
echolocation in both light and dark conditions while flying within a
tunnel.

Newborn bats appear to pick up echolocation rapidly. In their 2003
paper to the Journal of Neurophysiology, Marianne Vater and five
co-workers report that two-week-old Mustached bat (Pteronotus parnellii)
pups were capable of spontaneously producing CF and FM signals. Dr Vater
and her colleagues also report that the ability of these bats to utilize
DSC was evident from about four weeks old! It seems that, as well as a
rather rapid development of echolocation calls, the call structure can
vary according to geography. A 2003 study by Fanni Aspetsberger at the
University of Cape Town and two colleagues, found that echolocation
calls of Little Free-tailed bats (Chaerephon pumilus) in the Amani
Nature Reserve of Tanzania, were of a lower frequency and had longer
gaps between pulses than in those individuals of the same species living
in South Africa. These differences are probably related to differences
in feeding ecology between the populations.

Bats are not the only animals that use echolocation to find their way
about and locate food. Echolocation is perhaps best known in the
Odontoceti (toothed whales), especially the Delphinidae (dolphins). In
the case of dolphins, sounds (in the form of rapid, high pitched clicks)
of about 120 kHz are generated in the nasal sacs, after which the melon
(the bony surface of the skull) focuses the sound into a narrow band and
projects it forwards. Returning echoes are received by the pan bone of
the lower jaw, and are then transmitted to the middle ear by fatty
tissue located just behind the jaw; from the ear the sound is
transmitted to the brain. (Photo: Bats
aren't the only mammals to make use of echolocation. Members of the
dolphin family, which includes the Killer whale, also use sound to
locate prey.)

The observation that sound travels four-and-a-half times faster in
water than in air suggests that the dolphin’s brain must be extremely
well adapted to make sense of the returning echoes, which arrive more
rapidly than they do for bats. This may explain why dolphins tend to
transmit each click after receiving the echo from the previous one. Some
authors have even suggested that the dolphins’ echolocation may have a
healing effect on humans. It has been postulated that the ultrasound
emitted by dolphins may have a mechanical and/or electro-mechanical
effect on the endocrine (hormone) system, positively stimulating it and
providing some relief from certain psychological and psychosomatic
illnesses. Research into this idea by Karsten Brensing, Katrin
Linke and Dietmar Todt at the University of Berlin, however, rejected the idea
that dolphins exhibit a behaviour that leads to patients being exposed
to ultrasound in doses comparable to those in medical treatments. (Back
to Menu)

Q: How is Moth Evolution Linked to Bat Echolocation?

A: Inextricably,
in some species. In his fascinating contribution to Recent Advances in
the Study of Bats, James Fullard at the University of Toronto in Canada
reports that the auditory system of Noctuid (Cutworm) moths evolved as a
direct result of predation by bats. Dr Fullard notes that this species
doesn’t use sound socially, but their tympanal (middle ear) organs are
often most sensitive to the frequency of calls emitted by echolocating
(and, therefore, hunting) bats. In other words, these moths have evolved
to hear the bats that feed on them coming! Indeed, Fullard has
conducted numerous studies into how moth hearing has evolved in relation
to the echolocative calls of the bats that feed on them. Over the years,
he has come to realize that most moths that fly in the same airspace as
hunting bats avoid being eaten by using their ears, which are syntonic
with the hunting calls of bats – that is to say, moths have evolved to
hear in the same sound range that bats have evolved to hunt with. In one
particular study, published in the Proceedings of the Royal Society of
London back in 2001, Fullard reports on the predation of moths by
Hawaiian Hoary bats (Lasiurus cinereus semotus) on the Hawaiian island
of Kaua’i, observing that the endemic Hawaiian Cutworm moths (Haliophyle
euclidias) were preferentially eaten by this bat, compared to other
endemic and introduced species. Fullard concluded that this moth --
which has hearing that is less sensitive to bat calls than the other
species of moth he looked at -- suffers higher predation because it is
drawn away from its normal habitat, enticed by the man-made lights that
are now favoured hunting grounds for bats.

Intriguingly, although the hearing of certain moths (especially
noctuid moths) is related to bat predation, the emergence of nocturnal
activity in moths seems unrelated to bat activity. In a 2000 paper,
James Fullard reported on the day-flying butterflies in Polynesian
bat-free habitat, comparing the activity of three species of nymphalid
(Brush-footed) butterfly on the bat-free Pacific island of Moorea with
three nymphalids in Queensland, Australia (where bats actively prey on
moths). Fullard found that nocturnal flight activity and the number
of active individuals did not differ significantly between the two
locations, leading him to conclude that living in a bat-free environment
did not produce nocturnal flight in these insects. This is strange,
especially considering that during the daytime, moths are at risk of
predation from birds and are competing for nectar with butterflies; thus
exploiting the night-time in a habitat where there are no major
nocturnal predators (i.e. bats) would seem the best course of action.

Fullard considered three possible reasons for the daytime activity
seen in the butterflies from bat-free habitats: that bats weren’t
important nocturnal predators; that the insects are somehow constrained
to the day by some physiological reason; or that the Moorean butterflies
haven’t spent enough time in genetic isolation. The first of these
suggestions is unlikely – bats are a major predator of insects in almost
every forested region of the world, and it seems doubtful that they
weren’t an important influence in this study. The second and third ideas
are both plausible – of these, Dr Fullard considers that the second is
most probable and these insects are constrained by some physiological
parameter (either temperature or light).

Subsequent research has documented hearing
sensitivities at the range of bat calls in other, non-noctuid, moths. Annemarie
Surlykke at Denmark University and four colleagues, for
example, report that the ear of Drepanid (Hooktip) moths is tuned to
ultrasonic frequencies between 30 and 65 kHz. Such an observation
suggests that drepanid hearing resembles that of other moths, in that
the main function is bat detection. (Back to Menu)

Q: What Bat Species are Found in the UK?

A: There are currently 16 species of bat known from
the UK, six of which are considered -- conservationally-speaking --
“Vulnerable”, four are “Rare”, two are “Endangered” and only four are
not threatened to a sufficient extent to warrant adding to a
conservation list. The 16 UK species and their conservational status are
as follows (S = Scotland; W = Wales; I = Ireland; E = England):

Up until the January 1990, when it was declared extinct in Britain,
the Common Mouse-eared bat (Myotis myotis) was the 17th member of this
list. Since its removal, there have been occasional sightings of
hibernating Myotis myotis in the UK. Such sightings have,
however, been
of single individuals and I'm not aware of any evidence to show that the
species has begun recolonising the British Isles. Additionally, the
regions above should be considered tentative. In Wales, for example,
there are only isolated records of P. nauthusii, E. serotinus, and
M. bechsteini. Similarly, species identification is easier for some bats
than others, and some species are very difficult to tell apart. M. brandtii vs. M. mystacinus or
P. pipistrellus vs. P. pygmaeus, for
example, are often recorded as simply 'brandt/whiskered' and 'pipistrelle',
respectively. (Back to Menu)

Q: What it the “Pipistrelle Split”?

A: It was the realization that,
what was once thought to be a single species (Pipistrellus pipistrellus),
is actually two different species (P. pipistrellus and P. pygmaeus) that
probably diverged some five to ten million years ago. Bat workers had
known for some time that the Common pipistrelle (P. pipistrellus)
occurred in two apparently different forms; differences in appearance
and in the peak frequencies of their echolocation calls had been
documented. It was not until the early 1990s, however, that anyone
actually set about categorizing these differences and looking into the
possibility that they might be different species.

The first investigations were conducted by Gareth Jones at Bristol
University and one of his students in 1992 – the results showed that not
only did the two different forms use different maternity roosts, but
they also had different call frequencies: 45 kHz and 55 kHz. Subsequent
experiments, conducted by John Altringham, Gareth Jones and Kirsty Park,
looked at the mating behaviour of the two “phonic” (i.e. with different
frequency calls) pipistrelles. Prof Altringham and his team found that
the two types did not share mating roosts but were thus considered to be
reproductively isolated (i.e. they don’t interbreed with one another). A
plethora of ensuing papers reported various differences in the
morphology and feeding ecology of the two phonic types and, eventually,
DNA analysis was conducted in a bid to find out whether the two types
were actually separate species. The analysis was conducted by a team led
by Liz Barratt and Mike Bruford at London Zoo using a sample of wing
tissue. The results, published in the journal Nature in 1997,
showed that the two types couldn’t interbreed – in other words, they
were different species. Thus, the two types were reclassified as
different species: P. pygmaeus (the Brown, Pygmy, Soprano or 55 kHz pipistrelle)
and P. pipistrellus (the Common, Bandit or 45 kHz pipistrelle).

Unfortunately, while DNA analysis has shown that we now have two
species where we originally considered there to be only one, this
doesn’t mean that they are easily separated. Although there are
differences in overall appearance and behaviour of the two pipistrelles
they are still VERY similar. The Common pipistrelle has a darker face
and ears than the Pygmy pipistrelle, giving the appearance of a mask and
leading to some giving it the vernacular name “Bandit pipistrelle”. There
is also no single feature that is significantly different in their
teeth arrangement or biometric measurements (i.e. the size of various
body parts) and moreover, some individuals overlap in the peak
frequencies of their echolocation calls, making identification in the
field complicated. Just to muddy the waters even further, in his
A Guide
to the Identification of Pipistrelle Bats, Henry Schofield of the
Vincent Wildlife Trust in Herefordshire notes that “there is suggestion
that the overall appearance of the two species may vary
geographically…making them easier to separate in some areas of the
country than in others”. Henry also states that some people think that
the bats “smell” different, although this has yet to be subjected to
rigorous scientific study!

Unfortunately, separating the "Common"
and "Pygmy" Pipistrelle is not as easy in the field as it is in the
genetics laboratory!

As something of a sideline, it seems that our British pipistrelles
are not the only ones subject to tangled taxonomics! There is
considerable debate in the US about the taxonomic status of Hollister’s
bat (Myotis occultus). Over the last few decades, bat
researchers have been trying to figure out whether M. occultus is a discrete species, or
whether it is actually a synonym of the Little Brown bat (Myotis
lucifugus). In 1999, Michael Bogan at the University of New
Mexico and three of his colleagues carried out tests on the two species
and, based on the high similarities between the two and little
divergence from the Hardy-Weinberg equilibrium (the idea that
populations are in “genetic equilibrium”), concluded that the two bats
are nominal taxa and M. occultus should be regarded as a
subspecies of M. lucifugus. If the two bats were that
similar, however, why keep M. occultus as a subspecies? Why
not just say that the two are the same species? Well, despite the
genetic similarities, there are still rather obvious morphological
differences between the two bats – thus, Dr Bogan and his team suggested
M. occultus as a subspecies.

Data published in the Journal of Mammology by a team of American
geneticists again looked at the discombobulated taxonomy of these little
bats. This time, a team led by Antoinette Piaggio at the San
Francisco State University looked at two genes from mitochondrial DNA
(that is only inherited along the maternal bloodline) and found that M. occulatus
represents “an evolutionarily distinct monophyletic lineage” – in other
words, Piaggio and his team support the idea that M. occultus and
M. lucifugus are separate species. The jury is, however,
still out and, as
is now commonplace for most things of taxonomic ilk, it is up to the
reader to decide whose line of evidence he or she finds most compelling!
(Back to Menu)

Q: Is the Expression “Blind as a Bat” Justified?

A: The short answer is: No! Indeed, while most bats
(i.e. the microbats) have monochromatic vision (are colour blind), some
(i.e. megabats) may see in colour.

Flying foxes (megabats) have exceptionally large eyes, and --
considering the lack of feeding-orientated echolocation in these bats --
vision obviously plays an important role in finding food, avoiding
obstacles and perhaps finding a mate. Royal Melbourne Institute of
Technology biologist Mal Graydon published a fascinating summary of
Fruit bat vision in the June 1997 issue of Friends of Bats newsletter. In
his article, Dr Graydon notes that Flying foxes can very easily adapt
to their daylight surrounds, a skill afforded by the rapid contraction
of the iris.

Studies by the esteemed scientist Gerhard Neuweiler during the 1970s
concluded that Fruit bats have a visual acuity far superior to ours in
dim light. Moreover, it seems that the “bob and sway” observed in flying
megabats may be related to “two-eye analyses”. Using both eyes when
assembling information about a visual target allows the brain an
opportunity to compare two sets of information about the object (one
from each optic nerve) and produce a final image that is more accurate
with respect to textures, distance and shape. (Photo: Grey-Headed Flying
fox, Pteropus poliocephalus.)

Assessing the presence of colour vision in animals is tricky, and
normally relies on a combination of retinal scans and psychological
tests. Humans have two types of cells on their retina: rods and cones. Rods
are used to detect changes in light levels and contrast (i.e. serve
a monochromatic function), while cones are used to collect and transmit
information about colour. Rods and cones have been documented in many
different species and are, generally speaking, considered to have
much-the-same function across the species barrier. In Fruit bats, the
retina is almost entirely covered with rod cells. Strewn in
amongst these rods, however, are cells that don’t look entirely rod-like, nor do
they look entirely cone-like, although they look more like cones than
rods. The number of these cells on the retina is minor, but it does
leave the question of whether megabats have full colour processing
ability open to debate. More recent studies on the retina of two
megabats by a team of eight scientists, led by Daryi Wang at Academic
Sinica in Taiwan, has revealed that these bats have the gene associated
with detection of red light, which the researchers suggest might aid the
bat in discriminating between fruit and foliage. Ergo, the results
suggest at least some colour processing by these Fruit bats.

The role that echolocation plays in object avoidance and hunting in
the microbats has reduced the need for high visual acuity. Consequently,
with few exceptions, microbat eyes are proportionally smaller than their
megabat kin and their roughly spherical lenses suggest a short focal
distance, good depth of field and probable hyperopia (far-sightedness). A
series of experiments by the late Martin Eisentraut in the 1970s found
that Brown Long-eared bats (Plecotus auritus) were able to discriminate
different targets, but not different shapes (i.e. they could tell the
difference between black squares and white squares, but not a circle and
a cross). Dr. Eisentraut’s experiments were, however, carried out in
bright light, which may have affected the outcome. Various studies have
revealed that bat vision works better in dim light and studies on Phyllostomid (New World Leaf-nosed) bats, by Roderick Suthers (currently
at the Indiana University) and his co-workers, in the late 1960s and
early 1970s revealed more sophisticated discrimination of patterns in
these chiropterans. Intriguingly, papers in the journal Animal Behaviour
in 1981 and 1983 looking at the escape behaviour of the Geffroy’s
Tail-less bat (Anoura geoffroyi), reported that this microbat used
visual cues alone when selecting an escape route from the experimental
setup, discarding acoustic cues.

The above studies and on-going experiments on microbats suggest that
they may rely on vision more than was originally considered. A
fascinating study by Johan Eklof of Goteborg University in Sweden and
Gareth Jones at Bristol University published in Animal Behaviour during
2003 revealed that visual cues were more important than acoustic ones to
foraging P. auritus. The scientists even found evidence for spatial
memory in their subjects – the bats were observed to hover over places
where dishes of worms were once placed but had subsequently been
removed, suggesting that they remembered that there had once been food
there. Elkof and Jones observed more feeding attempts at dishes that
provided only visual cues, compared with those that provided only sonar
cues, suggesting that not only were the bats able to locate food by
sight, they also seemed to ‘prefer’ using visual cues rather than
acoustic ones.

Recently, it has become clear that many bat
species seem to have a sensitivity to ultraviolet (UV) light, which is
more abundant at dawn and dusk. Indeed, York Winter at the University of
Munich in Germany and two of her co-workers were able to demonstrate
that the Long-tongued Nectar bat (Glossophaga soricina) is sensitive to
UV down to a wavelength of 310 nm. York and her colleagues -- Jorge
Lopez at the Universidad de San Carlos in Guatemala and Otto von
Helversen of Erlangen University in Germany -- also conducted
behavioural experiments that revealed a sensitivity in the green (max.
510 nm) and UV (above 365 nm) spectra. Moreover, the team found that the
same photoreceptor (light-sensing cell) is responsible for both peaks
(i.e. in the green and the UV) – this is interesting because in all
rodents and marsupials (pouched mammals) where colour vision has been
established, there is a separate receptor to deal with UV light. Indeed,
the mechanism described for UV vision by Dr Winter and he co-workers has
never been demonstrated in intact mammal visual systems before! (Back to
Menu)

Q: What Should I do if I Find an Injured Bat?

A: Ideally, phone the
National Bat Helpline, run by
the Bat Conservation Trust (BCT), on (0845) 1300 228 or e-mail the BCT
on enquiries@bats.org.uk. The Helpline is,
however, only staffed Monday
to Friday from 9am to 5.30pm (GMT). Ergo, should you find a grounded bat
-- or should your family feline bring one home for you -- there are a
few things you can do to make the creature a little more comfortable
while you contact the necessary authorities. The Bat Conservation Trust
has a list of guidelines on their website
regarding the caring for of grounded and/or injured bats. Alternatively,
you could contact your nearest bat group, who may offer advice over the
telephone or send a qualified bat handler out to you. There are upwards
of 90 volunteer bat groups in the UK, 33 of which have websites – a list
of local bat groups can be found on the Bat Conservation Trust’s
website. The following is a summary of the information provided by the
BCT. First, and foremost is DO NOT pick the bat up with your bare hands, use decent gloves
(gardening gloves are excellent) or a cloth.

Care for a grounded bat:

1. If the bat has injuries and you are going to be keeping the bat for
longer than a couple of hours, prepare some suitable housing.

2. Housing should be something such as a shoebox or large margarine tub,
with sufficient air holes (but no gaps larger than 5mm / one-quarter in.).

3. Line the housing with kitchen towel or soft cloth and place it in a
warm spot – a dark airing cupboard is ideal.

4. Offer water regularly on a small clean paintbrush, cotton bud or in a
teaspoon – don’t put a pot of water in with the bat.

5. Bats may be enticed to feed on small meaty chunks of cat food.

It is very important that you do NOT harm the
bat – the law protects all species of UK bat and, under the Wildlife and
Countryside Act of 1981, causing injury to them (or their roosts) is a
criminal offence. (Back to Menu)